1. Field of Invention
This invention relates to defending transport aircraft such as civilian airliners against attack by shoulder-fired missiles (MANPADS) employing combined infrared and ultraviolet sensitive seekers and IR-only seekers.
2. Introduction
Heat-seeking MANPADS (Man Portable Air Defense Systems) missiles such as the FIM-92 Stinger, shoulder launched anti-aircraft missile and various similar foreign versions (ref: “Raytheon Electronic Systems FIM-92 Stinger low-altitude surface-to-air missile system family”, Jane's Land-Based Air Defence, Oct. 13, 2003, (See http://www.ianes.com/defence/air_forces/news/jlad/jlad001013—2_n.shtml) present a critical and pressing terrorist threat to commercial air transport aircraft. The most vulnerable phases of flight are during landing approach and immediately after takeoff. Many landing approach profiles require prolonged flight at low altitudes over populated areas. Nevertheless, although the stakes are high, the probability that any particular transport would ever be attacked is very low. For society, this situation produces a cost-effectiveness conundrum. A desirable missile defense system for transports must operate continuously, effectively, reliably, and economically.
Various systems have been proposed to defeat infrared (IR) missile threats. Pyrotechnic flares are traditionally used for this purpose, but have short effective time durations. Routinely dispensing flares to draw possible MANPADS missiles away from a transport is clearly unacceptable. Dispensing flares or recoverable decoys when an attack is detected requires a sophisticated and costly missile attack sensing system. Recurring false alarms would likely cause unacceptable hazards from flares to people and property.
Tethered decoys have also been proposed. Non-predeployed recoverable decoys must be deployed quickly after receiving a warning, which places stringent requirements on the tether line and requires a complex release and recovery systems. Decoys must also radiate considerable IR power, which limits operating duration or requires significant power-carrying capacity by the tether. Fuel burning decoys must be refueled and battery-operated decoys need to be recharged or replaced often, requiring costly and time-consuming ground operations. Other issue relate to handling potentially hazardous materials at passenger terminals.
Active jamming systems, including Directed Infrared Counter Measures (DIRCM), have also been strongly advocated. A typical proposed DIRCM system comprises three principal components: (1) a missile attack warning system; (2) data processing system; and, (3) a directed high-power laser in a gimbaled turret. At an estimated average cost of $1M or greater per aircraft, plus large recurring operating and maintenance costs, it would clearly be an extremely expensive proposition to outfit the entire 7000-aircraft US commercial transport fleet with active systems. The DIRCM system is more appropriate to larger aircraft, and produces severe cost performance penalties on smaller aircraft, such as the B-737 and Airbus-300 series, which make up the large majority of aircraft in the US fleet.
In short, an urgent need exists for an alternative cost-effective, long-term solution to the threat MANPADS missile systems to air transport aircraft fleets.
My invention comprises a non-consumable passive defense system that promises significantly lower initial and lifetime costs as well as effective protection against terrorist MANPADS attacks on commercial transport aircraft. The mere presence of such a system would likely dissuade attacks because the probability of a successful kill becomes very small while the probability of detecting the terrorists would be still significant. The simplicity and low cost of my system can lead to quick deployment throughout the commercial transport fleet and thus provide prompt defense and protection for our transport aircraft fleets an early date.
Newer generations of MANPADS shoulder launched missiles may employ imaging technologies to sense the contrast between sky background light or radiation and parts of an aircraft shaded particularly at shorter wavelengths, such as background UV. In particular, the lower surfaces of aircraft are shaded from both direct sunlight and from background sky brightness. Vertical surfaces of aircraft often have significant contrast from the sky background as well. As a counter, counter-measure, IR seeking missiles with such capability would veer into or toward darker sensed areas.
The present invention combines means for reducing or eliminating that UV contrast with IR counter-MANPADS methods, such as DIRCM, dispensed flares, and, in particular, towed decoys.
Initial MANPADS versions suffered from several problems, including locking on to bright IR sources, such as decoy flares and, notably, the sun. Later design generations incorporated a seeker or homing head that uses a dual IR and UV scanner. Two detector materials are used, one responsive to IR in the 3.5-5.0 micrometer wavelength band and the other responsive to UV in the 0.3-0.4 micrometer band (essentially the 0.32-0.4 micrometer “UVA” band). Such homing heads allow discrimination against background clutter and decoy flares, which appear as point sources of UV as well as of IR radiation. Sources that produce both UV and IR are ignored or severely down-weighted by the seeker sensor systems, preventing or reducing the likelihood that the missile will be attracted to them. More recent MANPADS improvements incorporate IR and UV focal plane array (FPA) sensors. Initial target acquisition and tracking are performed by IR seeker sensor systems. As the missile nears the intended target, perhaps 1 second (about 600 m) before impact, the seeker sensor system modifies the missile trajectory by biasing a steering signal toward UV-dark areas contrasting with the sky background.
Obviously, these newer MANPADS missile systems function more effectively in the daytime. At night, both the sky and the aircraft are dark and there is no contrast steering bias produced. During daylight hours a steering bias towards darker or shaded areas, in reality might be better described as a bias against brighter areas.
Countermeasures include reducing or reversing (making the shadowed parts of the aircraft brighter than the sky background) the UVA contrast between the shaded portions of the airframe. Zeroing or eliminating UVA contrast effectively cloaks the airframe. Reversing the contrast (making the airframe brighter) produces a steering bias away from the airframe. These countermeasures can be applied selectively to protect the more vulnerable parts of the airframe. Other potential countermeasure involves reducing the apparent visual size of the airframe in the UVA band by selectively illuminating shaded areas. This would delay activation of the UV target adaptive guidance (TAG) acquisition mode until it is too late to effectively steer the missile.
Although invisible to humans, the daylight sky is rather uniformly bright in the UVA band, even under overcast conditions due to scattering of UV photons by air molecules in the upper atmosphere. The overall indirect UVA sky radiation is approximately equal to direct UVA sunlight reaching the ground under clear sky conditions. Except within a relatively narrow ring around the sun, the UVA sky radiance is rather uniform across the sky hemisphere. The average UVA sky radiance does not depend strongly on the solar zenith angle. Under overcast conditions, even when the sun is completely obscured, lower, but nevertheless significant, numbers of UVA photons reach the surface via multiple scattering events within the clouds. The UVA sky radiance under overcast conditions is weaker and more uniform throughout the sky hemisphere than under a clear sky.
Under clear sky conditions, the average UVA sky radiance is typically about 2-3 Wm−2sr−1. Thus, to be invisible (i.e., zero contrast with the background sky) at UVA wavelengths, the shaded portion of an airframe would have to provide this level of artificial UVA radiance either by reflected or direct radiation. 3.4 Wm−2sr−1 corresponds to a radiant exitance of about 1 W per sq.ft. In this situation, cloaking 1000 sq.ft. of shaded aircraft surface would require less than 1 kW total illumination from an external UVA lamp.
Reflection from low clouds and the earth's surface can significantly reduce the UVA contrast between the sky and shaded aircraft surfaces and thus the required level of make-up radiance. The UVA albedo of the earth ranges from about 8% over forested areas to 90% or more over low clouds and ice- or snow covered regions. Over the latter regions the undersides of aircraft may be brighter than the sky background. Over forests, under clear skies, reflected direct solar and sky background illumination can reduce the required level of make-up radiation by 15%, somewhat reducing the required make-up radiant exitance over solidly forested and similar surfaces. Significant reductions in make-up radiant exitance requirements are expected over more general and varied surface types. Under overcast skies the required make-up radiant exitance will be significantly lower than this for all earth surface types.
The worst-case contrast between the sky and shaded aircraft surfaces occurs at high altitudes under clear skies over tropical forested regions within a ring close to the sun when it is directly overhead (zenith angle ˜0°). The peak UVA sky radiance looking directly at the sun is an order of magnitude greater than the average radiance over the sky hemisphere. Even so, 75% of the sky hemisphere has a UVA radiance less than 3 Wm−2sr−1 for solar zenith angle greater than 20° and more than 90% of the sky is less than that for zenith angles greater than 30°. Over the continental US, the solar zenith angle is always greater than 25°. Lower solar zenith angles only occur in tropical areas near noon when the sun is directly overhead. However, it is very unlikely that a missile attack would be launched essentially vertically into the noonday sun at an aircraft more-or-less directly overhead. MANPADS missiles do not work well when fired at targets near the sun—if their seekers can acquire them at all. Early, IR-only MANPADS tended to lock on to the sun instead of an aircraft flying a transit momentarily occluding the sun. Later-generation models with seeker sensor systems strongly biased away from high UVA-radiance levels have mitigated such sun miscues. In addition, even at low approach speeds, a transport aircraft at an altitude corresponding to the maximum MANPADS operating range will transit, in just a few seconds, the bright UVA ring immediately surrounding the sun in the atmosphere. Therefore, practical levels of UVA radiation will effectively cloak and/or bias MANPADS missiles away from a targeted aircraft. It is not necessary to cloak, shaded aircraft surfaces from the higher UVA sky background levels surrounding the sun.